Dummy drugThe latest Dutch study, which was due to run until 2020, was being carried out across 11 hospitals in the Netherlands, including the Amsterdam University Medical Centre.

In total, 93 women were given sildenafil (the non-brand name for Viagra) while the remaining 90 were given a dummy drug or placebo.

Twenty babies developed lung problems after birth - three in the placebo group and the rest in the treatment group.

Eleven in the sildenafil group died from lung complications.

Prof Zarcko Alfirevic, from the University of Liverpool, who led part of the UK research into sildenafil in pregnancy that found no benefit in terms of improving baby growth, said: "This finding in the Dutch study is unexpected.

"We need to be careful at this point to find out more.

"It needs a thorough investigation because the complications were not seen in the two other, similar trials that have already been done in the UK and Australia and New Zealand."